Base-metal thermocouple tolerances and their utility in real-world measurements

Abstract

The reference functions and tables for base-metal thermocouples, which relate temperature and electro-motive-force, are specified by thermocouple type, rather than by alloy composition. The common thermocouple types—E, J, K, N, and T—are required to meet the specifications within defined tolerance bands; referred to as either limits-of-error (ASTM, US) or class (IEC, UK and Europe). In this requirement hides a fundamental problem: the tolerances are a manufacturing specification. Despite this, the tolerances are often used as a proxy for expected behaviour. However, these thermocouple types are subject to rapid and irreversible changes after short exposure times to modest temperatures. Thus, tolerance statements are poor predictors of in-use behaviour. This paper initially reviews the development of the base-metal thermocouples before identifying many of the metallurgical processes that lead to departures from the accepted tolerance bands. Focus is specifically given to the processes that occur in several type K and N thermocouples, which exemplify the changes common in the other base-metal thermocouples. From these results, it is shown that the use of traditional calibration techniques can be a futile exercise. Comparisons are then made between as-received thermocouples and those given thermal treatments, that have been designed to allow meaningful calibration while also inhibiting drift. These results show that thermal treatments and calibration are a practicable means to minimise drift over typical periods of use while offering small uncertainties, well within the span of manufacturing tolerances. Lastly it is suggested manufacturers give more information on the testing used to establish conformance with tables and reference functions and that they follow standardised testing procedures.